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NTC THERMISTOR DESIGN GUIDEF O R D I S C R E T E C O M P O N E N T S & P R O B E S

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Special Services

OUR MISSION: Through teamwork, to achieve industry’s confidence as the

highest quality producer of temperature sensors in the world.

I N D U S T R Y ’ S PA R T N E R I N Q U A L I T Y A N D P E R F O R M A N C E ™

NTC THERMISTOR DESIGN GUIDE FOR DISCRETE COMPONENTS & PROBES What is a thermistor? ..........................................................................................3

How to use a thermistor .....................................................................................5

Why use a thermistor? ........................................................................................6

How do I use a Thermistor? ...............................................................................7

How much resistance do I need? .....................................................................8

What’s a curve and which curve do I choose? .............................................9

What is Thermal Time Constant? (Mil-PRF 23648) ..................................10

What is Thermal Dissipation Constant? ....................................................... 11

What is Self Heating? ........................................................................................ 11

How do I design a probe? .................................................................................12

Insulation Properties ..........................................................................................13

Conversion Tables ................................................................................................14

Frequently Asked Questions .............................................................................15

New Products .......................................................................................................16

How small can you make a part? ...................................................................17

SPECIAL SERVICES ......................................................... 18

Since 1977, Quality Thermistor, Inc. has designed and manufactured PTC and NTC thermistors

of superior quality. Millions of QTI TM Brand thermistor temperature probes have been used for

mission critical applications from deep below the oceans’ surface to the outer reaches of space.

Our state-of-the-art manufacturing facility located in Boise, Idaho combined with our high-volume

assembly plant in Mexico ensure no project is to small or large for us to accommodate.

This NTC thermistor design guide has been thoughtfully prepared to address some of the most common

temperature related questions facing design engineers. If you have additional questions, please feel free

to contact us. We would be happy to work with you on your application.

CONTENTS

Qualified Test LabTo ensure the quality of our QTI

brand thermistors, Quality

Thermistor has an extensive test

lab for a wide range of testing

services. In addition, this facility is available for customers for

the following services:

• Power burn-in

• Temperature cycling

• Moisture testing

• Shock and vibration testing

• Temperature characterization

• Space-level screening

• QCI Military testing

Custom DesignWith a full staff of experienced temperature application

engineers, Quality Thermistor can provide custom design services

at any step along the design process. Experts in temperature

measurement, compensation, and control, Quality Thermistor

engineers can work with your in-house engineers or contractors,

or as a full-support design team to solve your application.

• Components

• Probes

• Boards

• Systems

• Control and signal

conditioning

Private LabelingThe QTI brand is recognized in many industries for high-quality

manufacturing and measurement accuracy and reliability.

However, in situations where private labeling is required, Quality

Thermistor will provide components with no label or with your

label to ensure the integrity of your branding strategy.

• Your design, your label

• Our design, your label

• Your design, the QTI label

AssemblyQuality Thermistor offers expert, timely component and board

assembly services in our well-equipped Tecate, Mexico, facility.

In addition, to ensure product is delivered on time, the facility’s

capability is mirrored at our Idaho plant.

• Highly-trained assemblers

• High-volume production

• Competitive prices

• Probe assembly

• PTC and NTC devices

An NTC thermistor is a semiconductor made from metalic

oxides, pressed into a small bead, disk, wafer, or other shape,

sintered at high temperatures, and then coated with epoxy or

glass. The resulting device exhibits an electrical resistance that

has a very predictable change with temperature.

Thermistors are widely used for temperature monitoring, control

and compensation. They are extremely sensitive to temperature

change, very accurate and interchangeable. They have a wide

temperature envelope and can be hermetically sealed for use in

humid environments.

The term “thermistor” originated from the descriptor THERMally

sensitive ResISTOR. The two basic types of thermistors are the

Negative Temperature Coefficient (NTC) and Positive Temperature

Coefficient (PTC).

Thermistors are available as surface mount or radial and axial

leaded packages. The leaded parts can then be either over

molded or housed in a variety of shapes and materials.

Although this design guide focuses on NTC (Negative

Temperature Coefficient), thermistors are also available in

Positive Temperature Coefficients.

therm·is·tor Pronunciation: ther-mis-ter, thur-muh-ster

Origin: 1935–40

Function: noun

Etymology: thermal resistor An electrical resistor whose resistance varies

rapidly and predictably with temperature and as a result can be used to measure temperature.

THERMISTOR STYLES

Axial Leaded (PTC)

RTH42

RTH22 PTC

QTG12 PTC

OTG10 PTC

Surface Mount

1206

0805 NTC & PTC

0603

0402

NTC Radial Leaded

QTMC

QTLC

Bare Die

QTC11 NTC

QTC11 PTC







• Power burn-in













• Components

• Probes

• Boards

• Systems


conditioning
















• Probe assembly


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What is a thermistor?Specia l Services

ThermistorFrom Wikipedia, the free encyclopedia

A thermistor is a type of resistor used to measure temperature

changes, relying on the change in its resistance with chang-

ing temperature. The Thermistor was first invented by Samuel

Ruben in 1930, and has U.S. Patent #2,021,491.

If we assume that the relationship between resistance and

temperature is linear (i.e. we make a first-order approximation),

then we can say that:

∆R = kΔT

Where

ΔR = change in resistance t

ΔT = change in temperature

k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the

sign of k. If k is positive, the resistance increases with increas-

ing temperature, and the device is called a positive temperature

coefficient (PTC) thermistor. If k is negative, the resistance

decreases with increasing temperature, and the device is called a

negative temperature coefficient (NTC) thermistor. Resistors that

are not thermistors are designed to have the smallest possible

k, so that their resistance remains almost constant over a wide

temperature range.

Steinhart Hart equationIn practice, the linear approximation (above) works only over a

small temperature range. For accurate temperature measure-

ments, the resistance/temperature curve of the device must be

described in more detail. The Steinhart-Hart equation is a widely

used third-order approximation:

where a, b and c are called the Steinhart-Hart parameters, and

must be specified for each device. T is the temperature in Kelvin

and R is the resistance in ohms. To give resistance as a function

of temperature, the above can be rearranged into:

where and

The error in the Steinhart-Hart equation is generally less than

0.02°C in the measurement of temperature. As an example, typi-

cal values for a thermistor with a resistance of 3000 Ω at room

temperature (25°C = 298.15 K) are:

a =1.40 x 10-3

b =2.37 x 10-4

c =9.90 x 10-8

Conduction modelMany NTC thermistors are made from a pressed disc or cast chip

of a semiconductor such as a sintered metal oxide. They work

because raising the temperature of a semiconductor increases

the number of electrons able to move about and carry charge

- it promotes them into the conducting band. The more charge

carriers that are available, the more current a material can con-

duct. This is described in the formula:

I = n • A • v • e I = electric current (ampere)

n = density of charge carriers (count/m3)

A = cross-sectional area of the material (m2)

v = velocity of charge carriers (m/s)

e = charge of an electron

The current is measured using an ammeter. Over large changes

in temperature, calibration is necessary. Over small changes in

temperature, if the right semiconductor is used, the resistance

of the material is linearly proportional to the temperature. There

are many different semiconducting thermistors and their range

goes from about 0.01 Kelvin to 2,000 Kelvins (-273.14°C to

1,700°C). QTI range -55 to 300ºC.

Most PTC thermistors are of the "switching" type, which means

that their resistance rises suddenly at a certain critical tempera-

ture. The devices are made of a doped polycrystalline ceramic

containing barium titanate (BaTiO3) and other compounds. QTI

manufactures silicon based PTC thermistors that are .7%/ºC.

Thermistor Symbol

Part Number Bead Dia. Resistance Tolerance

QT06002-524 .023" 10,000 +/- 0.1ºC (0ºC to 70º)

QT06002-525 .023" 10,000 +/- 0.2ºC (0ºC to 70º)

NANO TUBE 0.023" MAX OD epoxy fi lled polyimide tube with insulated #38 AWG solid nickel leads, parallel bonded, 6" (76.2 mm)

Part Number Bead Dia. Resistance

QTMB-14 .038" 10,000

QTMB16 .038" 15,000

MINI BEAD 0.038" MAX OD epoxy coated bead with #34 AWG Poly-nylon insulated bifi lar leads, twisted pair, 6" (152.4 mm). Tolerance +/- 0.2ºC (0ºC to 70º)

C O N F I N E D S P A C E T H E R M I S T O R S & T E M P E R A T U R E P R O B E S

• Exceptionally fast thermal response time

• Suitable for smaller temperature probe housings

• Custom and semi-custom products may be specifi ed

• Available in point matched and interchangeable tolerances


QT06002-529 .031" 2,252

QT06002-530 .031" 3,000

QT06002-531 .031" 5,000

QT06002-532 .031" 10,000

MICRO TUBE 0.031" MAX OD epoxy fi lled polyimide tube with polyurethane nylon insulated #32 AWG solid copper leads, twisted pair, 6" (152.4mm). Tolerance +/- 0.2º (0ºC to 70º)


QT06002-526 .037" 2,252

QT06002-533 .037" 3,000

QT06002-527 .037" 5,000

QT06002-528 .037" 10,000

MINI TUBE 0.037" MAX OD epoxy fi lled polyimide tube with polyurethane nylon insulated #32 AWG solid copper leads, twisted pair, 6" (152.4mm). Tolerance +/- 0.2º (0ºC to 70º)

RESISTANCE

Temp(ºC) 2,252 3,000 5,000 10,000

0 7,355 9,798 16,330 32,660

5 5,720 7,620 12,700 25,400

10 4,481 5,970 9,950 19,900

15 3,538 4,713 7,855 15,710

20 2,813 3,747 6,245 12,490

25 2,252 3,000 5,000 10,000

30 1,815 2,417 4,029 8,058

35 1,471 1,960 3,266 6,532

40 1,199 1,598 2,663 5,326

45 984 1,310 2,184 4,368

50 811 1,081 1,801 3,602

55 672 896 1,493 2,986

60 560 746 1,244 2,488

65 469 625 1,041 2,082

66 453 603 1,005 2,010

67 437 582 971 1,941

68 422 563 938 1,875

69 408 544 906 1,812

70 394 525 876 1,751

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NTC Thermistors

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How Smal l Can You Make a Thermistor?What is a thermistor?

NTC Thermistors

Quality Thermistor, Inc. a leader in thermistor innovation

is pleased to announce the Thermal Bridge. The Thermal

Bridge incorporates a bridge resistor with the thermistor

providing a more linear signal for conditioning. By incorporating

a bridge resistor with a thermistor in a single precision assem-

bly, temperature sensing is implemented without the need for

calibration, potentionmeters, precision external components and

with no concern for clocking and bus issues.

Temperature is determined by a ratio of the

input versus output voltage across the sensor

allowing inexpensive and precise tempera-

ture measurement capability for nearly any

Microprocessor based

system. With widely available embedded

mixed signal processors and A-D converters,

Design Engineers can easily condition the

non-linear signal of NTC thermistors.

FEATURES AND BENEFITS OF USING THE

THERMAL BRIDGE:

• Operating temperature range of -55

to 150ºC

• Accuracy up to +/-.2ºC

from 0–70ºC

– Up to +/-1ºC from -55 to 100ºC

– Up to +/-1.5ºC from -55 to 150ºC

• Available in many probe configurations or as a circuit

board mounted component

• High stability with no calibration required

• Long sensor life-span

• Dynamic response for ease of measurement

• Wide operating voltage range, up to 48 VDC

• Monolithic thermistor sensor exhibits negligible

capacitance and inductance

• No error introduced due to noise, and random noise

self-cancels

• Low power consumption, 170uW maximum

-55

20

0

40

60

80

100

-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125

Vout

(%Vi

n)C

The NTC thermistor is best suited for precision temperature mea-

surement. The PTC is best suited for temperature compensation.

The NTC thermistor is used in three different modes of operation,

which services a variety of applications.

Resistance-Versus-Temperature Mode - By far the

most prevalent. These circuits perform precision temperature

measurement, control and compensation. Unlike the other appli-

cations this method depends on the thermistor being operated in

a “zero-power” condition. This condition implies that there is no

self-heating.

The resistance across the sensor is relatively high in comparison

to an RTD element, which is usually in the hundreds of ohms

range. Typically, the 25°C rating for thermistors is from 10Ω up

to 10,000,000Ω. The housing of the thermistor varies as the

requirements for a hermetic seal and ruggedness, but in most

cases, there are only two wires going to the element. This is pos-

sible because of the resistance of the wire over temperature is

considerably lower than the thermistor element. And typically

does not require compensation because of the overall resistance.

Current-Over-Time Mode – This depends on the dissipa-

tion constant of the thermistor package as well as element’s

heat capacity. As current is applied to a thermistor, the package

will begin to self-heat. If the current is continuous, the resis-

tance of the thermistor will start to lessen. The thermistor cur-

rent-time characteristics can be used to slow down the affects

of a high voltage spike, which could be for a short duration. In

this manner, a time delay from the thermistor is used to prevent

false triggering of relays.

This type of time response is relatively fast as compared to

diodes or silicon based temperature sensors. In contrast, ther-

mocouples and RTD’s are equally as fast as the thermistor, but

they don’t have the equivalent high level outputs.

Voltage-Versus-Current Mode - Voltage-versus-current

applications use one or more thermistors that are operated in a

self-heated condition. An example of this would be a flow meter.

The thermistor would be in an ambient self-heated condition.

The thermistor’s resistance is changed by the amount of heat

generated by the power dissipated by the element. Any change

in the media (gas/liquid) across the device changes the power

dissipation factor of the thermistor. The small size of the therm-

istor allows for this type of application to be implemented with

minimal interference to the system.

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NTC Thermistors


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NTC Thermistors

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New Products How To Use a Thermistor

erature mea-

mpensation.

of operation,

By far the

mperature

e other appli-

g operated in

t there is no

comparison

s of ohms

om 10Ω up

es as the

ut in most

t. This is pos-

perature is

nd typically

all resistance.

the dissipa-

element’s

the package

the resis-

mistor cur-

the affects

duration. In

ed to prevent

ared to

h

Resolution - Large change in resistance for a small

change in temperature

Another advantage of the thermistor is its relatively high resis-

tance. Thermistors are available with base resistances (at 25°

C) ranging from tens to millions of ohms. This high resistance

reduces the effect of resistance in the lead wires, which can

cause significant errors with low resistance devices such as

RTD’s. An example of this is the traditional RTD, which typically

requires 3-wire or 4-wire connections to reduce errors, caused

by lead wire resistance; 2-wire connections to thermistors are

usually adequate.

The thermistor has been used primarily for high-resolution mea-

surements over limited temperature ranges (-55° to 150°C). The

classic example of this would be a medical application where

the user is only concerned with body temperature. However,

widespread improvements in thermistor stability, accuracy, and

interchangeability have prompted increased usage of thermistors

in all types of industries.

CostThermistors are by far the most economical choice when it

comes to temperature sensors. Not only are they less expensive

to purchase, but also there are no calibration costs during instal-

lation or during the service life of the sensor. In addition, if

there is a failure in the field, interchangeable thermistors can be

swapped out without calibration.

SpeedDue to their small size, thermistors can respond very quickly to

slight changes in temperature. Caution must be taken when

designing probes because materials and mass play an important

role in the reaction time of the sensor. See section on “Thermal

Time Constant” and “How do I design a probe?” for further

details.

No Calibration RequiredProperly manufactured thermistors are aged to reduce drift

before leaving the factory. Therefore, thermistors can provide a

stable resistance output over long periods of time.

How does aging affect thermistor stability?“Thermometric drift” is a specific type of drift in which the

drift is the same amount of temperature at all temperatures of

exposure. For example, a thermistor that exhibits a -0.02°C shift

at 0°, 40° and 70°C (even though this is a different percentage

change in resistance in each case) would be exhibiting thermo-

metric drift. Thermometric drift: (1) occurs over time at varying

rates, based on thermistor type and exposure temperature, and

(2) as a general rule, increases as the exposure temperature

increases. Most drift is thermometric.

How do thermistors fail?

SILVER MIGRATION

This failure can occur when one or more of the following

three conditions are present: constant direct current bias, high

humidity, and electrolytes (disc/chip contamination). Moisture

finds its way into the thermistor and reacts with the contami-

nant. Silver (on the thermistor electrodes) turns to solution,

and the direct current bias stimulates silver crystal growth

across the thermistor element. The thermistor resistance

decreases, eventually reaching zero O (short) (probably the

most common failure mechanism).

MICRO CRACKS

Thermistors can crack due to improper potting materials if a

temperature change causes potting material to contract, crush-

ing the thermistor. The result is a thermistor that has erratic

resistance readings and is electrically “noisy.”

FRACTURE OF GLASS ENVELOPE

Typically caused by mishandling of thermistor leads, this failure

mechanism induces fractures in the glass coating at the lead/

thermistor interface. These cracks may propagate around the

thermistor bead resulting in a catastrophic upward shift in

resistance. Mismatching of epoxies or other bonding materials

may also cause this. Careful handling and the proper selection

of potting materials can eliminate this failure.

AGING OUT OF RESISTIVE TOLERANCE

If thermistors are exposed to high temperatures over time,

sometimes referred to as “aging,” their resistivity can change.

Generally the change is an upward change in resistivity, which

results in a downward change in temperature. Selecting the

proper thermistor for the temperature range being measured

can minimize the occurrence of this failure. Temperature

cycling may be thought of as a form of aging. It is the cumula-

tive exposure to high temperature that has the greatest influ-

ence on a thermistor component, not the actual temperature

cycling. Temperature cycling can induce shifts if the compo-

nent has been built into an assembly with epoxies or adhesives,

which do not match the temperature expansion characteristics

of the thermistor.

What happens if my application exceeds the temperature rating?

Intermittent temperature incursions above and below the oper-

ating range will not affect long-term survivability. Encapsulate

epoxy typically begins to break down at 150°C and the solder

attaching leads to the thermistor body typically reflows at about

180°C. Either condition could result in failure of the thermistor.

Are thermistors ESD sensitive?

Per MIL-DTL-39032E, Table 1, thermistors by definition are not

ESD sensitive.

What is the resolution of a thermistor?

There is no limit to the resolution of a thermistor. The limitations

are in the electronics needed to measure to a specified resolu-

tion. Limitations also exist in determining the accuracy of the

measurement at a specified resolution.

Are QTI thermistors RoHS compliant?

(What if I don’t want a lead free part?)

Quality Thermistor maintains two separate manufacturing lines

to meet the specific environmental needs of our customers. One

line is dedicated to RoHS compliance and the other is main-

tained for traditional tin/lead parts for military, aerospace and

medical applications.

Does the length of wire impact the accuracy of a thermistor?With a thermistor, you have the benefit of choosing a higher

base resistance if the wire resistance is a substantial percentage

of the total resistance. An example of this would be a 100-ohm

thermistor vs a 50,000 ohm thermistor with 10’ of 24 AWG wire.

Total wire resistance = 10’ x 2 wires x 0.02567 ohms per foot =

0.5134 ohms

The amount of drift over a period of time is dependent on the aging temperature. Please note that not all thermistor manufactures age at the same temperature so drift data may be different.

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NTC Thermistors NTC Thermistors

Frequently Asked Quest ions

S u m m e r 2 0 07

Why Use a Thermistor?

What is meant by “Interchangeability” or “Curve tracking”? A thermistor can be defined as having an interchangeability

tolerance of ±0.1°C over the range from 0° to 70°C. This means

that all points between 0° and 70°C, are within 0.1°C of the

nominal resistance values for that particular thermistor curve.

This feature results in temperature measurements accurate to

±0.1°C no matter how many different thermistors are substi-

tuted in the application.

What is meant by ‘”Point Matched”?

A standard thermistor is calibrated and tested at 25°C to a toler-

ance of ± 1%, 2%, 5% or ± 10%. Since these thermistors only

have one controlled point of reference or ‘point matched’ tem-

perature, the resistance at other temperatures are given by the

"Resistance vs. Temperature Conversion Tables" for the appropri-

ate material curve. The resistance value at any temperature is

the ratio factor times the resistance at 25°C.

In addition to the industry

standard of point matching

thermistors at 25°C, Quality

Thermistor can point match

to a specific temperature

range. Examples of this

would be the freezing point

of water (0°C) or human

body temperature (37°C).

AVAILABLE INTERCHANGEABLE TOLERANCES

0Cº to +70ºC

A3 = +/- 1ºC

B3 = +/- 0.5ºC

C3 = +/- 0.2ºC

D3 = +/- 0.1ºC

-20Cº to +50ºC

A2 = +/- 1ºC

B2 = +/- 0.5ºC

C2 = +/- 0.2ºC

0Cº to 100ºC

A4 = +/- 1°C

B4 = +/- 0.5°C

C4 = +/- 0.2°C

+20Cº to +90ºC

A5 = +/- 1ºC

B5 = +/- 0.5ºC

C5 = +/- 0.2ºC

+20Cº to +50ºC

A6 = +/- 1ºC

B6 = +/- 0.5ºC

C6 = +/- 0.2ºC

D6 = +/- 0.1ºC

Closed end tube withflange, ideal forrivet mounting.

EQUIVALENT TABLES Decimal/inches/mm

Standard Stud Terminal Stud Size Diameter Hole Dia. U.S. (metric) In. (mm) In. (mm)

#2 .0866 .090M2 (2.18) (2.29)

#4 .112 .118(M2,5) (2.84) (3.00)

#5 .125 .127(M3) (3.18) (3.23)

#6 .138 .146(M3,5) (3.51) (3.71)

#8 .164 .173(M4) (4.17) (4.39)

#10 .190 .198(M5) (4.83) (5.03)

1/4” .250 .270(M6) (6.35) (6.86)

5/16” .312 .330(M8) (7.92) (8.38)

3/8” .375 .385(M10) (9.53) (9.78)

1/2” .500 .520(M12) (12,7) (13.21)

5/8” .625 .650(M16) (15.88) (16.51)

3/4” .750 .810(M18) (19.05) (20.57)

Diameter Diameter Ohms per Ohms Size Inches mm 1000 ft per km

20 AWG 0.032 0.813 10.15 33.29

21 AWG 0.029 0.724 12.80 41.98

22 AWG 0.025 0.645 16.14 52.94

23 AWG 0.023 0.574 20.36 66.78

24 AWG 0.020 0.511 25.67 84.20

25 AWG 0.018 0.455 32.37 106.17

26 AWG 0.016 0.404 40.81 133.86

27 AWG 0.014 0.361 51.47 168.82

28 AWG 0.013 0.320 64.90 212.87

29 AWG 0.011 0.287 81.83 268.40

30 AWG 0.010 0.254 103.20 338.50

31 AWG 0.009 0.226 130.10 426.73

32 AWG 0.008 0.203 164.10 538.25

33 AWG 0.007 0.180 206.90 678.63

34 AWG 0.006 0.160 260.90 855.75

35 AWG 0.006 0.142 329.00 1,079.12

36 AWG 0.005 0.127 414.80 1,360.00

37 AWG 0.005 0.114 523.10 1,715.00

38 AWG 0.004 0.102 659.60 2,163.00

2.0 mm 0.008 0.203 169.39 555.61

1.8 mm 0.007 0.178 207.50 680.55

1.6 mm 0.006 0.152 260.90 855.75

1.4 mm 0.006 0.152 339.00 1,114.00

1.25 mm 0.005 0.127 428.20 1,404.00

1.12 mm 0.004 0.102 533.80 1,750.00

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NTC Thermistors

How Do I Use a Thermistor?

NTC Thermistors

Convers ion Tables

S u m m e r 2 0 07

With an NTC thermistor, resistance decreases as the temperature

rises. One main factor in determining how much resistance you

need at 25°C is to calculate how much resistance you will have at

your critical temperature range.

If the total wire resistance is a substantial percentage of the

resistance change at your critical temperature range, you should

consider increasing your base resistance at 25°C.

Determine if the resistance change at your critical temperature is

large enough to compensate for any other errors in your systems

design. If not, you should increase your base resistance at 25°C.

EXAMPLE –

1,000 Ω curve Z thermistor at 25°C

Between –29°C and –28° C, there is a resistance change of

990 ohms. Between 118° and 119° C, there is only a resistance

change of 1.1 ohms.

Resitance R/T

@25ºC Part# Curve

500 QTMC-1 Z

2,250 -7 Z

2,500 -8 Z

3,000 -9 Z

5,000 -11 Z

10,000 -14 Z

20,000 -19 Z

1,000 -27 Y

2,000 -28 Y

100,000 -43 V

Resitance R/T

@25ºC Part# Curve

1 Meg -65 P

9.8 Meg -70 R

100 -78 X

Halar- PVC- Tefl on- Poly- Tefzel THERMAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Maximum Continous

Rating (Cº) 105 135 105 135 260 150 200 200 260 150

Low Temperature (Cº) -50 -100 -60 -70 -200 -100 -200 -200 -200 -100

Non-Flammability Very Good Excellent Very Good Excellent Excellent Good Excellent Excellent Excellent Excellent

Solder Resistant Good Very Good Very Good Very Good Very Good Very Good Excellent Excellent Excellent Excellent

Smoke Moderate Slight Moderate Slight None Moderate None None None Slight

Halar- PVC- Tefl on- Poly- Tefzel ELECTRICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Volume Resistivity

(ohm-cm) 1012 1013 1016 2x1014 1018 5x1016 2x1018 1018 1012 1016

Dielectric Strength (1 mil fi lm)

VPM, 1/8” thick 350 490 350 450 430 400 430 420 430 400

Dielectric Constant 5.70 2.60 3.50 7.70 2.06 3.13 2.00 2.40 2.00 2.60

Dissipation Factor

(1 kHz) .09 .002 .03 .02 .0002 .001 0.4 .001 .0002 .0008

Capacitive Frequency

Stability Fair Excellent Good Poor Excellent Good Excellent Excellent Excellent Excellent

Halar- PVC- Tefl on- Poly- Tefzel MECHANICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE 1.68 (67% Density (gm/cc) 1.36 1.68 1.48 1.76 2.15 1.24 2.18 polyimide) 2.20 1.70

Tensile, psi 4,000 7,000 15,000 6,000 4,000 10,000 2,700 17,000 2,500 6,500

Elongation, % 250 200 50 250 300 100 250 75 225 100-400

Abrasion Resistance Fair Fair Good Excellent Good Excellent Good Excellent Good Excellent

Cut-through Resistance Good Good Excellent Excellent Fair Excellent Fair Excellent Fair Excellent

Bondability Good Good Good Good Good Excellent Good

Halar- PVC- Tefl on- Poly- Tefzel ENVIRONMENTAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE 100 200 approx.100 Nuclear Radiation Fair megarads Fair Excellent Fair Good Fair megarads Fair megarads

UV Radiation Fair Excellent Fair Excellent Excellent Fair Excellent Excellent Excellent Excellent

Halar- PVC- Tefl on- Poly- Tefzel CHEMICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Water Absorbtion 0.7% .01% .06% .04% .03% .05% .01% .8% .01% .1%

Acids Good Excellent Good Very Good Excellent Good Excellent Fair Excellent Excellent

Alkali Good Excellent Poor Very Good Excellent Good Excellent Fair Excellent Excellent

Alcohol Fair Excellent Fair Very Good Excellent Fair Excellent Very Good Excellent Excellent

Cleaning Solvents Slight

(tri-chlor, carbon, tetr) Swell Excellent Good Very Good Excellent Crazes Excellent Very Good Excellent Excellent

Aliphatic Hydrocarbons Slight

(gasoline, kersosene) Swell Excellent Fair Very Good Excellent Good Excellent Very Good Excellent Excellent

Aromatic Hydrocarbons Slight

(benzene, toulene) Swell Excellent Fair Very Good Excellent Crazes Excellent Very Good Excellent Excellent

Long Term Stability Fair Excellent Good Very Good Excellent Very Good Excellent Excellent Excellent Excellent

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NTC Thermistors

How Much Resistance Do I Need?

NTC Thermistors

Wire Insulat ion Propert ies

S u m m e r 2 0 07

If you recall our definition of a thermistor (An electrical resistor

making use of a semiconductor whose resistance varies sharply

in a very predictable manner with temperature.) We can use the

Stein-Hart Hart equation to predict how the thermistor reacts

to temperature. If we plot these points on a graph, it forms a

repeatable curve. Thermistor manufacturers can alter the chem-

istry of a thermistor, thereby changing the slope of a curve.

Your curve selection should be based on how steep the curve is

for your critical temperature range, size constraints and the

target resistance value. Since a thermistor is based on bulk

resistivity, the size of the sensor my not be feasible for your

application. Unlike the RTD and Thermocouples, thermistors do

not have industry standards for their curves. However, most

thermistor manufacturers have curves that are similar. An

example of this is Quality Thermistors ‘Z’ curve it’s by far the

most common curve in the industry and most major thermistor

manufacturers have a very similar curve offerings.

Another problem with selecting material based on thermal con-

ductivity alone is that if the mass of highly conductive probe

housing can actually act like a heat sink and pull additional heat

out of the system. This can obviously create measuring inac-

curacies.

To offset this, you can combine different materials while design-

ing your probe. A low thermally conductive housing with a

small highly conductive probe tip is a good solution.

In some cases, your application may require a slow thermal time

response. An example of this would be an outdoor sign that dis-

plays the temperature. A large over molded probe will insulate

the thermistor and even out quick fluctuations in temperature

changes.

CONFINED SPACE

Due to a thermistors miniature

size, they can be potted into

almost any size housing. Currently,

the smallest available thermistor

is 0.023” max diameter. Hollow-

tube rivets, set screws, hypodermic

needles and direct epoxy attach are

some common methods for con-

fined space thermistor applications.

LIQUID

For liquid applications, it’s best

to use a threaded probe. Possibly,

with some type of elastomeric seal

like an o-ring. QTI also offers a

complete line of NPT probe hous-

ings. Some applications require

over molding the thermistor into

the plastic housing of the product.

Another option is to use a glass

encapsulated bead. It provides a

hermetic seal that is as close to

‘waterproof’ as Mother Nature will

let us. Remember the Titanic?

GAS/AIR

Gas and air applications have a

variety of choices. Probes can

be surface mounted in the flow

stream or they can be projected

into the air stream by means of a

closed or open-end tube. When

measuring gas or air under pres-

sure, we recommend using some

type of thread/o-ring combination.

SURFACE

By far the most common method

for surface measurement is the

ring lug. Due to the small size of

the thermistor element, it can be

potted into most ring lug barrels.

Be careful that the wire gauge

does not exceed the inside dimen-

sion of the barrel. Another option

for surface measurement is direct

attachment of a thermistor using a

stainless steel disc.

Curve Z

Curve W

Curve X

Curve Y

Curve V

Curve S

Curve M

Curve R

4

R M

ult

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er

Temperature ( ºC)

3

2

1

0

0 10 20 30 40 50 60 70

RESISTANCE VALUE IS ALSO A FUNCTION OF CURVE

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NTC Thermistors

How Do I Design A Probe?

NTC Thermistors

What's a Curve And Which Curve Do I Choose?

S u m m e r 2 0 07

THERMAL DISSIPATION CONSTANT

The thermal dissipation constant of a thermistor is the power

required to raise the thermistors body temperature by 1°C. The

dissipation constant is expressed in units of mW/°C (milliWatts

per degree Centigrade).

Dissipation Constant can be affected by:

✔ Mass of the thermistor probe

✔ How the probe and sensor are mounted

✔ Thermal dynamics of the environment

The dissipation constant is an important factor in applica-

tions that are based on the self-heating effect of thermistors.

Specifically, the change in resistance of the thermistor due to

change in dissipation constant can be used to monitor levels or

flow rates of liquids or gasses. As an example as the flow rate

increases, the dissipation constant of the thermistor in a fluid

path will increase and the resistance will change and can be

correlated to the flow rate.

Stated another way, the dissipation constant is a measure of the

thermal connection of the thermistor to its surroundings. It is

generally given for the thermistor in still air, but sometimes in

well-stirred oil.

THERMAL TIME CONSTANT

The thermal time constant is the time required for a thermis-

tor to change to 63.2 percent of the total difference between

its initial and final body temperature when subjected to a step

function change in temperature under zero power conditions.

The United States Department of Defense has a very specific

method for measuring the thermal time response of a thermistor

(see Mil Spec 23648)

Place thermistors in a still air controlled chamber (chamber tem-

perature: 25°C ±1°C) with a minimum volume of 1,000 times the

thermistor body and test fixture.

✔ Self heat the thermistor to 75°C. Allow 15 minutes (max-

imum) for stabilization of thermistors.

✔ Prepare to measure time from the instant the power is

cut to the time the bridge indicator passes through the

null point (43.4°C)

✔ Record this time: This is the time constant of the thermis-

tor is register a 63.3% change in temperature.

That’s right, the DoD Specification for thermal time response is how fast a thermistor can react to a 32° change!

Some thermistor manufacturers choose to use a 50°C change.

Be sure and consult the product specifications when making a

comparison.

THERMAL CONDUCTIVITY

Heat moves through a material at a specific rate. The rate it

travels depends on the material itself: some materials allow

heat to move quickly through them, some materials allow heat

to move very slowly through them. Below is a list of different

materials and how they conduct heat.

MATERIAL THERMAL CONDUCTIVITY (W/M K)

Silver - Best 429

Copper (pure) 401

Gold 317

Aluminum (pure) 237

Brass (70Cu-30Zn) 110

Titanium 21.9

316 Stainless Steel 13.4

PEEK plastic 1.75

Thermally Conductive Epoxy 1.25

UHMW plastic 0.42

Beware of choosing a probe material based solely on conductivity. Corrosion resistance, cost, strength and machineability are all key factors.

SELF-HEATING EFFECTS

When current flows through a thermistor, it generates heat,

which raises the temperature of the thermistor above that of its

environment. This of course will cause an error in measurement

if not compensated for. Typically, the smaller the thermistor, the

lower the amount of current needed to self-heat.

The electrical power input to the thermistor is just

PE = IV

where I is current and V is the voltage drop across the thermis-

tor. This power is converted to heat, and this heat energy is

transferred to the surrounding environment. The rate of transfer

is well described by Newton's law of cooling:

PT = K(T (R) - T0)

where T(R) is the temperature of the thermistor as a function of

its resistance R, T0 is the temperature of the surroundings, and

K is the dissipation constant, usually expressed in units of mil-

liwatts per °C. At equilibrium, the two rates must be equal.

PE = PT

The current and voltage across the thermistor will depend on

the particular circuit configuration. As a simple example, if the

voltage across the thermistor is held fixed, then by Ohm's Law

we have I = V / R and the equilibrium equation can be solved for

the ambient temperature as a function of the measured resis-

tance of the thermistor:

T0 = T (R) -

The dissipation constant is a measure of the thermal connection

of the thermistor to its surroundings. It is generally given for

the thermistor in still air, and in well-stirred oil. Typical values

for a small glass bead thermistor are 1.5 mw/°C in still air and

6.0 mw/°C in stirred oil. If the temperature of the environ-

ment is known beforehand, then a thermistor may be used to

measure the value of the dissipation constant. For example, the

thermistor may be used as a flow rate sensor, since the dissipa-

tion constant increases with the rate of flow of a fluid past the

thermistor.

V 2

KR

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NTC Thermistors

What is Thermal T ime Constant? (Mil-PRF 23648 & Mil-PRF 32192),

NTC Thermistors

What is Thermal Diss ipat ion Constant?

S u m m e r 2 0 07

What is Self Heat ing?

If you recall our definition of a thermistor (An electrical resistor

making use of a semiconductor whose resistance varies sharply

in a very predictable manner with temperature.) We can use the

Stein-Hart Hart equation to predict how the thermistor reacts

to temperature. If we plot these points on a graph, it forms a

repeatable curve. Thermistor manufacturers can alter the chem-

istry of a thermistor, thereby changing the slope of a curve.

Your curve selection should be based on how steep the curve is

for your critical temperature range, size constraints and the

target resistance value. Since a thermistor is based on bulk

resistivity, the size of the sensor my not be feasible for your

application. Unlike the RTD and Thermocouples, thermistors do

not have industry standards for their curves. However, most

thermistor manufacturers have curves that are similar. An

example of this is Quality Thermistors ‘Z’ curve it’s by far the

most common curve in the industry and most major thermistor

manufacturers have a very similar curve offerings.

Another problem with selecting material based on thermal con-

ductivity alone is that if the mass of highly conductive probe

housing can actually act like a heat sink and pull additional heat

out of the system. This can obviously create measuring inac-

curacies.

To offset this, you can combine different materials while design-

ing your probe. A low thermally conductive housing with a

small highly conductive probe tip is a good solution.

In some cases, your application may require a slow thermal time

response. An example of this would be an outdoor sign that dis-

plays the temperature. A large over molded probe will insulate

the thermistor and even out quick fluctuations in temperature

changes.

CONFINED SPACE

Due to a thermistors miniature

size, they can be potted into

almost any size housing. Currently,

the smallest available thermistor

is 0.023” max diameter. Hollow-

tube rivets, set screws, hypodermic

needles and direct epoxy attach are

some common methods for con-

fined space thermistor applications.

LIQUID

For liquid applications, it’s best

to use a threaded probe. Possibly,

with some type of elastomeric seal

like an o-ring. QTI also offers a

complete line of NPT probe hous-

ings. Some applications require

over molding the thermistor into

the plastic housing of the product.

Another option is to use a glass

encapsulated bead. It provides a

hermetic seal that is as close to

‘waterproof’ as Mother Nature will

let us. Remember the Titanic?

GAS/AIR

Gas and air applications have a

variety of choices. Probes can

be surface mounted in the flow

stream or they can be projected

into the air stream by means of a

closed or open-end tube. When

measuring gas or air under pres-

sure, we recommend using some

type of thread/o-ring combination.

SURFACE

By far the most common method

for surface measurement is the

ring lug. Due to the small size of

the thermistor element, it can be

potted into most ring lug barrels.

Be careful that the wire gauge

does not exceed the inside dimen-

sion of the barrel. Another option

for surface measurement is direct

attachment of a thermistor using a

stainless steel disc.

Curve Z

Curve W

Curve X

Curve Y

Curve V

Curve S

Curve M

Curve R

4

R M

ult

ipli

er

Temperature ( ºC)

3

2

1

0

0 10 20 30 40 50 60 70

RESISTANCE VALUE IS ALSO A FUNCTION OF CURVE

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NTC Thermistors

How Do I Design A Probe?

NTC Thermistors

What's a Curve And Which Curve Do I Choose?

S u m m e r 2 0 07

With an NTC thermistor, resistance decreases as the temperature

rises. One main factor in determining how much resistance you

need at 25°C is to calculate how much resistance you will have at

your critical temperature range.

If the total wire resistance is a substantial percentage of the

resistance change at your critical temperature range, you should

consider increasing your base resistance at 25°C.

Determine if the resistance change at your critical temperature is

large enough to compensate for any other errors in your systems

design. If not, you should increase your base resistance at 25°C.

EXAMPLE –

1,000 Ω curve Z thermistor at 25°C

Between –29°C and –28° C, there is a resistance change of

990 ohms. Between 118° and 119° C, there is only a resistance

change of 1.1 ohms.

Resitance R/T

@25ºC Part# Curve

500 QTMC-1 Z

2,250 -7 Z

2,500 -8 Z

3,000 -9 Z

5,000 -11 Z

10,000 -14 Z

20,000 -19 Z

1,000 -27 Y

2,000 -28 Y

100,000 -43 V

Resitance R/T

@25ºC Part# Curve

1 Meg -65 P

9.8 Meg -70 R

100 -78 X

Halar- PVC- Tefl on- Poly- Tefzel THERMAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Maximum Continous

Rating (Cº) 105 135 105 135 260 150 200 200 260 150

Low Temperature (Cº) -50 -100 -60 -70 -200 -100 -200 -200 -200 -100

Non-Flammability Very Good Excellent Very Good Excellent Excellent Good Excellent Excellent Excellent Excellent

Solder Resistant Good Very Good Very Good Very Good Very Good Very Good Excellent Excellent Excellent Excellent

Smoke Moderate Slight Moderate Slight None Moderate None None None Slight

Halar- PVC- Tefl on- Poly- Tefzel ELECTRICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Volume Resistivity

(ohm-cm) 1012 1013 1016 2x1014 1018 5x1016 2x1018 1018 1012 1016

Dielectric Strength (1 mil fi lm)

VPM, 1/8” thick 350 490 350 450 430 400 430 420 430 400

Dielectric Constant 5.70 2.60 3.50 7.70 2.06 3.13 2.00 2.40 2.00 2.60

Dissipation Factor

(1 kHz) .09 .002 .03 .02 .0002 .001 0.4 .001 .0002 .0008

Capacitive Frequency

Stability Fair Excellent Good Poor Excellent Good Excellent Excellent Excellent Excellent

Halar- PVC- Tefl on- Poly- Tefzel MECHANICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE 1.68 (67% Density (gm/cc) 1.36 1.68 1.48 1.76 2.15 1.24 2.18 polyimide) 2.20 1.70

Tensile, psi 4,000 7,000 15,000 6,000 4,000 10,000 2,700 17,000 2,500 6,500

Elongation, % 250 200 50 250 300 100 250 75 225 100-400

Abrasion Resistance Fair Fair Good Excellent Good Excellent Good Excellent Good Excellent

Cut-through Resistance Good Good Excellent Excellent Fair Excellent Fair Excellent Fair Excellent

Bondability Good Good Good Good Good Excellent Good

Halar- PVC- Tefl on- Poly- Tefzel ENVIRONMENTAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE 100 200 approx.100 Nuclear Radiation Fair megarads Fair Excellent Fair Good Fair megarads Fair megarads

UV Radiation Fair Excellent Fair Excellent Excellent Fair Excellent Excellent Excellent Excellent

Halar- PVC- Tefl on- Poly- Tefzel CHEMICAL PVC E-CTFE Mylar Kynar PFA Sulfone FEP Kapton TFE ETFE Water Absorbtion 0.7% .01% .06% .04% .03% .05% .01% .8% .01% .1%

Acids Good Excellent Good Very Good Excellent Good Excellent Fair Excellent Excellent

Alkali Good Excellent Poor Very Good Excellent Good Excellent Fair Excellent Excellent

Alcohol Fair Excellent Fair Very Good Excellent Fair Excellent Very Good Excellent Excellent

Cleaning Solvents Slight

(tri-chlor, carbon, tetr) Swell Excellent Good Very Good Excellent Crazes Excellent Very Good Excellent Excellent

Aliphatic Hydrocarbons Slight

(gasoline, kersosene) Swell Excellent Fair Very Good Excellent Good Excellent Very Good Excellent Excellent

Aromatic Hydrocarbons Slight

(benzene, toulene) Swell Excellent Fair Very Good Excellent Crazes Excellent Very Good Excellent Excellent

Long Term Stability Fair Excellent Good Very Good Excellent Very Good Excellent Excellent Excellent Excellent

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NTC Thermistors

How Much Resistance Do I Need?

NTC Thermistors

Wire Insulat ion Propert ies

S u m m e r 2 0 07

What is meant by “Interchangeability” or “Curve tracking”? A thermistor can be defined as having an interchangeability

tolerance of ±0.1°C over the range from 0° to 70°C. This means

that all points between 0° and 70°C, are within 0.1°C of the

nominal resistance values for that particular thermistor curve.

This feature results in temperature measurements accurate to

±0.1°C no matter how many different thermistors are substi-

tuted in the application.

What is meant by ‘”Point Matched”?

A standard thermistor is calibrated and tested at 25°C to a toler-

ance of ± 1%, 2%, 5% or ± 10%. Since these thermistors only

have one controlled point of reference or ‘point matched’ tem-

perature, the resistance at other temperatures are given by the

"Resistance vs. Temperature Conversion Tables" for the appropri-

ate material curve. The resistance value at any temperature is

the ratio factor times the resistance at 25°C.

In addition to the industry

standard of point matching

thermistors at 25°C, Quality

Thermistor can point match

to a specific temperature

range. Examples of this

would be the freezing point

of water (0°C) or human

body temperature (37°C).

AVAILABLE INTERCHANGEABLE TOLERANCES

0Cº to +70ºC

A3 = +/- 1ºC

B3 = +/- 0.5ºC

C3 = +/- 0.2ºC

D3 = +/- 0.1ºC

-20Cº to +50ºC

A2 = +/- 1ºC

B2 = +/- 0.5ºC

C2 = +/- 0.2ºC

0Cº to 100ºC

A4 = +/- 1°C

B4 = +/- 0.5°C

C4 = +/- 0.2°C

+20Cº to +90ºC

A5 = +/- 1ºC

B5 = +/- 0.5ºC

C5 = +/- 0.2ºC

+20Cº to +50ºC

A6 = +/- 1ºC

B6 = +/- 0.5ºC

C6 = +/- 0.2ºC

D6 = +/- 0.1ºC

Closed end tube withflange, ideal forrivet mounting.

EQUIVALENT TABLES Decimal/inches/mm

Standard Stud Terminal Stud Size Diameter Hole Dia. U.S. (metric) In. (mm) In. (mm)

#2 .0866 .090M2 (2.18) (2.29)

#4 .112 .118(M2,5) (2.84) (3.00)

#5 .125 .127(M3) (3.18) (3.23)

#6 .138 .146(M3,5) (3.51) (3.71)

#8 .164 .173(M4) (4.17) (4.39)

#10 .190 .198(M5) (4.83) (5.03)

1/4” .250 .270(M6) (6.35) (6.86)

5/16” .312 .330(M8) (7.92) (8.38)

3/8” .375 .385(M10) (9.53) (9.78)

1/2” .500 .520(M12) (12,7) (13.21)

5/8” .625 .650(M16) (15.88) (16.51)

3/4” .750 .810(M18) (19.05) (20.57)

Diameter Diameter Ohms per Ohms Size Inches mm 1000 ft per km

20 AWG 0.032 0.813 10.15 33.29

21 AWG 0.029 0.724 12.80 41.98

22 AWG 0.025 0.645 16.14 52.94

23 AWG 0.023 0.574 20.36 66.78

24 AWG 0.020 0.511 25.67 84.20

25 AWG 0.018 0.455 32.37 106.17

26 AWG 0.016 0.404 40.81 133.86

27 AWG 0.014 0.361 51.47 168.82

28 AWG 0.013 0.320 64.90 212.87

29 AWG 0.011 0.287 81.83 268.40

30 AWG 0.010 0.254 103.20 338.50

31 AWG 0.009 0.226 130.10 426.73

32 AWG 0.008 0.203 164.10 538.25

33 AWG 0.007 0.180 206.90 678.63

34 AWG 0.006 0.160 260.90 855.75

35 AWG 0.006 0.142 329.00 1,079.12

36 AWG 0.005 0.127 414.80 1,360.00

37 AWG 0.005 0.114 523.10 1,715.00

38 AWG 0.004 0.102 659.60 2,163.00

2.0 mm 0.008 0.203 169.39 555.61

1.8 mm 0.007 0.178 207.50 680.55

1.6 mm 0.006 0.152 260.90 855.75

1.4 mm 0.006 0.152 339.00 1,114.00

1.25 mm 0.005 0.127 428.20 1,404.00

1.12 mm 0.004 0.102 533.80 1,750.00

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NTC Thermistors

How Do I Use a Thermistor?

NTC Thermistors

Convers ion Tables

S u m m e r 2 0 07

Resolution - Large change in resistance for a small

change in temperature

Another advantage of the thermistor is its relatively high resis-

tance. Thermistors are available with base resistances (at 25°

C) ranging from tens to millions of ohms. This high resistance

reduces the effect of resistance in the lead wires, which can

cause significant errors with low resistance devices such as

RTD’s. An example of this is the traditional RTD, which typically

requires 3-wire or 4-wire connections to reduce errors, caused

by lead wire resistance; 2-wire connections to thermistors are

usually adequate.

The thermistor has been used primarily for high-resolution mea-

surements over limited temperature ranges (-55° to 150°C). The

classic example of this would be a medical application where

the user is only concerned with body temperature. However,

widespread improvements in thermistor stability, accuracy, and

interchangeability have prompted increased usage of thermistors

in all types of industries.

CostThermistors are by far the most economical choice when it

comes to temperature sensors. Not only are they less expensive

to purchase, but also there are no calibration costs during instal-

lation or during the service life of the sensor. In addition, if

there is a failure in the field, interchangeable thermistors can be

swapped out without calibration.

SpeedDue to their small size, thermistors can respond very quickly to

slight changes in temperature. Caution must be taken when

designing probes because materials and mass play an important

role in the reaction time of the sensor. See section on “Thermal

Time Constant” and “How do I design a probe?” for further

details.

No Calibration RequiredProperly manufactured thermistors are aged to reduce drift

before leaving the factory. Therefore, thermistors can provide a

stable resistance output over long periods of time.

How does aging affect thermistor stability?“Thermometric drift” is a specific type of drift in which the

drift is the same amount of temperature at all temperatures of

exposure. For example, a thermistor that exhibits a -0.02°C shift

at 0°, 40° and 70°C (even though this is a different percentage

change in resistance in each case) would be exhibiting thermo-

metric drift. Thermometric drift: (1) occurs over time at varying

rates, based on thermistor type and exposure temperature, and

(2) as a general rule, increases as the exposure temperature

increases. Most drift is thermometric.

How do thermistors fail?

SILVER MIGRATION

This failure can occur when one or more of the following

three conditions are present: constant direct current bias, high

humidity, and electrolytes (disc/chip contamination). Moisture

finds its way into the thermistor and reacts with the contami-

nant. Silver (on the thermistor electrodes) turns to solution,

and the direct current bias stimulates silver crystal growth

across the thermistor element. The thermistor resistance

decreases, eventually reaching zero O (short) (probably the

most common failure mechanism).

MICRO CRACKS

Thermistors can crack due to improper potting materials if a

temperature change causes potting material to contract, crush-

ing the thermistor. The result is a thermistor that has erratic

resistance readings and is electrically “noisy.”

FRACTURE OF GLASS ENVELOPE

Typically caused by mishandling of thermistor leads, this failure

mechanism induces fractures in the glass coating at the lead/

thermistor interface. These cracks may propagate around the

thermistor bead resulting in a catastrophic upward shift in

resistance. Mismatching of epoxies or other bonding materials

may also cause this. Careful handling and the proper selection

of potting materials can eliminate this failure.

AGING OUT OF RESISTIVE TOLERANCE

If thermistors are exposed to high temperatures over time,

sometimes referred to as “aging,” their resistivity can change.

Generally the change is an upward change in resistivity, which

results in a downward change in temperature. Selecting the

proper thermistor for the temperature range being measured

can minimize the occurrence of this failure. Temperature

cycling may be thought of as a form of aging. It is the cumula-

tive exposure to high temperature that has the greatest influ-

ence on a thermistor component, not the actual temperature

cycling. Temperature cycling can induce shifts if the compo-

nent has been built into an assembly with epoxies or adhesives,

which do not match the temperature expansion characteristics

of the thermistor.

What happens if my application exceeds the temperature rating?

Intermittent temperature incursions above and below the oper-

ating range will not affect long-term survivability. Encapsulate

epoxy typically begins to break down at 150°C and the solder

attaching leads to the thermistor body typically reflows at about

180°C. Either condition could result in failure of the thermistor.

Are thermistors ESD sensitive?

Per MIL-DTL-39032E, Table 1, thermistors by definition are not

ESD sensitive.

What is the resolution of a thermistor?

There is no limit to the resolution of a thermistor. The limitations

are in the electronics needed to measure to a specified resolu-

tion. Limitations also exist in determining the accuracy of the

measurement at a specified resolution.

Are QTI thermistors RoHS compliant?

(What if I don’t want a lead free part?)

Quality Thermistor maintains two separate manufacturing lines

to meet the specific environmental needs of our customers. One

line is dedicated to RoHS compliance and the other is main-

tained for traditional tin/lead parts for military, aerospace and

medical applications.

Does the length of wire impact the accuracy of a thermistor?With a thermistor, you have the benefit of choosing a higher

base resistance if the wire resistance is a substantial percentage

of the total resistance. An example of this would be a 100-ohm

thermistor vs a 50,000 ohm thermistor with 10’ of 24 AWG wire.

Total wire resistance = 10’ x 2 wires x 0.02567 ohms per foot =

0.5134 ohms

The amount of drift over a period of time is dependent on the aging temperature. Please note that not all thermistor manufactures age at the same temperature so drift data may be different. This chart shows typical drift when parts were aged at 125°C.

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NTC Thermistors NTC Thermistors

Frequently Asked Quest ions

S u m m e r 2 0 07

Why Use a Thermistor?

Quality Thermistor, Inc. a leader in thermistor innovation

is pleased to announce the Thermal Bridge. The Thermal

Bridge incorporates a bridge resistor with the thermistor

providing a more linear signal for conditioning. By incorporating

a bridge resistor with a thermistor in a single precision assem-

bly, temperature sensing is implemented without the need for

calibration, potentionmeters, precision external components and

with no concern for clocking and bus issues.

Temperature is determined by a ratio of the

input versus output voltage across the sensor

allowing inexpensive and precise tempera-

ture measurement capability for nearly any

Microprocessor based

system. With widely available embedded

mixed signal processors and A-D converters,

Design Engineers can easily condition the

non-linear signal of NTC thermistors.

FEATURES AND BENEFITS OF USING THE

THERMAL BRIDGE:

• Operating temperature range of -55

to 150ºC

• Accuracy up to +/-.2ºC

from 0–70ºC

– Up to +/-1ºC from -55 to 100ºC

– Up to +/-1.5ºC from -55 to 150ºC

• Available in many probe configurations or as a circuit

board mounted component

• High stability with no calibration required

• Long sensor life-span

• Dynamic response for ease of measurement

• Wide operating voltage range, up to 48 VDC

• Monolithic thermistor sensor exhibits negligible

capacitance and inductance

• No error introduced due to noise, and random noise

self-cancels

• Low power consumption, 170uW maximum

-55

20

0

40

60

80

100

-45 -35 -25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 115 125

Vout

(%Vi

n)C

The NTC thermistor is best suited for precision temperature mea-

surement. The PTC is best suited for temperature compensation.

The NTC thermistor is used in three different modes of operation,

which services a variety of applications.

Resistance-Versus-Temperature Mode - By far the

most prevalent. These circuits perform precision temperature

measurement, control and compensation. Unlike the other appli-

cations this method depends on the thermistor being operated in

a “zero-power” condition. This condition implies that there is no

self-heating.

The resistance across the sensor is relatively high in comparison

to an RTD element, which is usually in the hundreds of ohms

range. Typically, the 25°C rating for thermistors is from 10Ω up

to 10,000,000Ω. The housing of the thermistor varies as the

requirements for a hermetic seal and ruggedness, but in most

cases, there are only two wires going to the element. This is pos-

sible because of the resistance of the wire over temperature is

considerably lower than the thermistor element. And typically

does not require compensation because of the overall resistance.

Current-Over-Time Mode – This depends on the dissipa-

tion constant of the thermistor package as well as element’s

heat capacity. As current is applied to a thermistor, the package

will begin to self-heat. If the current is continuous, the resis-

tance of the thermistor will start to lessen. The thermistor cur-

rent-time characteristics can be used to slow down the affects

of a high voltage spike, which could be for a short duration. In

this manner, a time delay from the thermistor is used to prevent

false triggering of relays.

This type of time response is relatively fast as compared to

diodes or silicon based temperature sensors. In contrast, ther-

mocouples and RTD’s are equally as fast as the thermistor, but

they don’t have the equivalent high level outputs.

Voltage-Versus-Current Mode - Voltage-versus-current

applications use one or more thermistors that are operated in a

self-heated condition. An example of this would be a flow meter.

The thermistor would be in an ambient self-heated condition.

The thermistor’s resistance is changed by the amount of heat

generated by the power dissipated by the element. Any change

in the media (gas/liquid) across the device changes the power

dissipation factor of the thermistor. The small size of the therm-

istor allows for this type of application to be implemented with

minimal interference to the system.

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NTC Thermistors


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NTC Thermistors

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New Products How To Use a Thermistor

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ThermistorFrom Wikipedia, the free encyclopedia

A thermistor is a type of resistor used to measure temperature

changes, relying on the change in its resistance with chang-

ing temperature. The Thermistor was first invented by Samuel

Ruben in 1930, and has U.S. Patent #2,021,491.

If we assume that the relationship between resistance and

temperature is linear (i.e. we make a first-order approximation),

then we can say that:

∆R = kΔT

Where

ΔR = change in resistance t

ΔT = change in temperature

k = first-order temperature coefficient of resistance

Thermistors can be classified into two types depending on the

sign of k. If k is positive, the resistance increases with increas-

ing temperature, and the device is called a positive temperature

coefficient (PTC) thermistor. If k is negative, the resistance

decreases with increasing temperature, and the device is called a

negative temperature coefficient (NTC) thermistor. Resistors that

are not thermistors are designed to have the smallest possible

k, so that their resistance remains almost constant over a wide

temperature range.

Steinhart Hart equationIn practice, the linear approximation (above) works only over a

small temperature range. For accurate temperature measure-

ments, the resistance/temperature curve of the device must be

described in more detail. The Steinhart-Hart equation is a widely

used third-order approximation:

where a, b and c are called the Steinhart-Hart parameters, and

must be specified for each device. T is the temperature in Kelvin

and R is the resistance in ohms. To give resistance as a function

of temperature, the above can be rearranged into:

where and

The error in the Steinhart-Hart equation is generally less than

0.02°C in the measurement of temperature. As an example, typi-

cal values for a thermistor with a resistance of 3000 Ω at room

temperature (25°C = 298.15 K) are:

a =1.40 x 10-3

b =2.37 x 10-4

c =9.90 x 10-8

Conduction modelMany NTC thermistors are made from a pressed disc or cast chip

of a semiconductor such as a sintered metal oxide. They work

because raising the temperature of a semiconductor increases

the number of electrons able to move about and carry charge

- it promotes them into the conducting band. The more charge

carriers that are available, the more current a material can con-

duct. This is described in the formula:

I = n • A • v • e I = electric current (ampere)

n = density of charge carriers (count/m3)

A = cross-sectional area of the material (m2)

v = velocity of charge carriers (m/s)

e = charge of an electron

The current is measured using an ammeter. Over large changes

in temperature, calibration is necessary. Over small changes in

temperature, if the right semiconductor is used, the resistance

of the material is linearly proportional to the temperature. There

are many different semiconducting thermistors and their range

goes from about 0.01 Kelvin to 2,000 Kelvins (-273.14°C to

1,700°C). QTI range -55 to 300ºC.

Most PTC thermistors are of the "switching" type, which means

that their resistance rises suddenly at a certain critical tempera-

ture. The devices are made of a doped polycrystalline ceramic

containing barium titanate (BaTiO3) and other compounds. QTI

manufactures silicon based PTC thermistors that are .7%/ºC.

Thermistor Symbol

Part Number Bead Dia. Resistance Tolerance

QT06002-524 .023" 10,000 +/- 0.1ºC (0ºC to 70º)

QT06002-525 .023" 10,000 +/- 0.2ºC (0ºC to 70º)

NANO TUBE 0.023" MAX OD epoxy fi lled polyimide tube with insulated #38 AWG solid nickel leads, parallel bonded, 6" (76.2 mm)


QTMB-14 .038" 10,000

QTMB16 .038" 15,000

MINI BEAD 0.038" MAX OD epoxy coated bead with #34 AWG Poly-nylon insulated bifi lar leads, twisted pair, 6" (152.4 mm). Tolerance +/- 0.2ºC (0ºC to 70º)

C O N F I N E D S P A C E T H E R M I S T O R S & T E M P E R A T U R E P R O B E S

• Exceptionally fast thermal response time

• Suitable for smaller temperature probe housings

• Custom and semi-custom products may be specifi ed

• Available in point matched and interchangeable tolerances


QT06002-529 .031" 2,252

QT06002-530 .031" 3,000

QT06002-531 .031" 5,000

QT06002-532 .031" 10,000

MICRO TUBE 0.031" MAX OD epoxy fi lled polyimide tube with polyurethane nylon insulated #32 AWG solid copper leads, twisted pair, 6" (152.4mm). Tolerance +/- 0.2º (0ºC to 70º)


QT06002-526 .037" 2,252

QT06002-533 .037" 3,000

QT06002-527 .037" 5,000

QT06002-528 .037" 10,000

MINI TUBE 0.037" MAX OD epoxy fi lled polyimide tube with polyurethane nylon insulated #32 AWG solid copper leads, twisted pair, 6" (152.4mm). Tolerance +/- 0.2º (0ºC to 70º)

RESISTANCE

Temp(ºC) 2,252 3,000 5,000 10,000

0 7,355 9,798 16,330 32,660

5 5,720 7,620 12,700 25,400

10 4,481 5,970 9,950 19,900

15 3,538 4,713 7,855 15,710

20 2,813 3,747 6,245 12,490

25 2,252 3,000 5,000 10,000

30 1,815 2,417 4,029 8,058

35 1,471 1,960 3,266 6,532

40 1,199 1,598 2,663 5,326

45 984 1,310 2,184 4,368

50 811 1,081 1,801 3,602

55 672 896 1,493 2,986

60 560 746 1,244 2,488

65 469 625 1,041 2,082

66 453 603 1,005 2,010

67 437 582 971 1,941

68 422 563 938 1,875

69 408 544 906 1,812

70 394 525 876 1,751

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NTC Thermistors

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How Smal l Can You Make a Thermistor?What is a thermistor?

NTC Thermistors

An NTC thermistor is a semiconductor made from metalic

oxides, pressed into a small bead, disk, wafer, or other shape,

sintered at high temperatures, and then coated with epoxy or

glass. The resulting device exhibits an electrical resistance that

has a very predictable change with temperature.

Thermistors are widely used for temperature monitoring, control

and compensation. They are extremely sensitive to temperature

change, very accurate and interchangeable. They have a wide

temperature envelope and can be hermetically sealed for use in

humid environments.

The term “thermistor” originated from the descriptor THERMally

sensitive ResISTOR. The two basic types of thermistors are the

Negative Temperature Coefficient (NTC) and Positive Temperature

Coefficient (PTC).

Thermistors are available as surface mount or radial and axial

leaded packages. The leaded parts can then be either over

molded or housed in a variety of shapes and materials.

Although this design guide focuses on NTC (Negative

Temperature Coefficient), thermistors are also available in

Positive Temperature Coefficients.

therm·is·tor Pronunciation: ther-mis-ter, thur-muh-ster

Origin: 1935–40

Function: noun

Etymology: thermal resistor An electrical resistor whose resistance varies

rapidly and predictably with temperature and as a result can be used to measure temperature.

THERMISTOR STYLES

Axial Leaded (PTC)

RTH42

RTH22 PTC

QTG12 PTC

OTG10 PTC

Surface Mount

1206

0805 NTC & PTC

0603

0402

NTC Radial Leaded

QTMC

QTLC

Bare Die

QTC11 NTC

QTC11 PTC







• Power burn-in













• Components

• Probes

• Boards

• Systems


conditioning
















• Probe assembly


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What is a thermistor?Specia l Services

For more information on Quality Thermistor, Inc., or on QTI brand thermistors, probes, and engineering services, contact Technical Support.

Quality Thermistor, Inc.2108 Century WayBoise, ID 83709

www.thermistor.com

800-554-4784 U.S.208-377-3373 Worldwide208-376-4754 [email protected]

QTI, Leach Guard, and Hydroguard are

trademarks of Quality Thermistor, Inc.

www.thermistor.com

NTC THERMISTOR DESIGN GUIDEF O R D I S C R E T E C O M P O N E N T S & P R O B E S

ntc thermistor design guide - hvaceducationaustralia.com · an ntc thermistor is a semiconductor...

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